Modern installation techniques provide for underground installation of services required for community infrastructure. Sewage, water, electricity, gas and telecommunication services are increasingly being placed underground for improved safety and to create more visually pleasing surroundings that are not cluttered with open services.
Currently, the most utilised method for underground works is to excavate an open cut trench. This is where a trench is cut from the top surface and after insertion of piping or optical cable is then back-filled. This method is reasonably practical in areas of new construction where the lack of buildings, roads and infrastructure does not provide an obstacle to this method. However, in areas supporting existing construction, an open cut trench provides obvious disadvantages, major disruptions to roadways and high possibility of destruction of existing infrastructure (i.e. previously buried utilities). Also, when an open cut trench is completed and backfilled the resultant shift in the ground structure rarely results in a satisfactory end result as the trench site often sinks. Open trenches are also unsafe to pedestrians and workers.
Another concept employed for underground works is that of boring a horizontal underground hole. Several methods employ this philosophy as it generally overcomes the issues of disruption to roads and infrastructure as described for open cut trenches however even these methods have their inherent problems.
One method is horizontal directional drilling (HDD). In this method a boring device is situated on the ground surface and drills a hole into the ground at an oblique angle with respect to the ground surface. A drilling fluid is typically flowed through the drill string, over the boring tool, and back up the borehole in order to remove cuttings and dirt. After the boring tool reaches a desired depth, the tool is then directed along a substantially horizontal path to create a horizontal borehole. After the desired length of borehole has been obtained, the tool is then directed upwards to break through to the surface. A reamer is then attached to the drill string, which is pulled back through the borehole, thus reaming out the borehole to a larger diameter. It is common to attach a utility line or other conduit to the reaming tool so that it is dragged through the borehole along with the reamer. A major problem with this method is that the steering mechanism is extremely inaccurate and unsuitable for applications on grade. The stop and start action utilised by the operator results in a bore that is not completely straight. The operator has no way of knowing exactly where the hole goes which can result in damage to existing utilities. This could pose a safety threat particularly if the services in the area are of a volatile nature.
Another method is the pilot displacement method. This method uses a drill string pushed into the ground and rotated by a jacking frame. A theodolite is focused along the drill string as a point of reference to keep the line on grade. This system is not accurately steered. The slant on the nose is pointed in the direction of intended steering. The position of the head is monitored through a total station with a grade and line set and measuring this point against a target mounted in the head of the pilot string. If the ground conditions are homogenous and the conditions absolutely perfect, it will produce a satisfactory bore. Unfortunately this is rarely the case. Ground conditions are generally variable the pilot tube will tend to steer towards whichever ground offers the least resistance irrespective of the direction in which you are the steering. As the drill strings are generally short, the time to drill is often slow with repeated connections making the process tedious. Once the bore reaches the reception shaft augers are attached and pulled back along the bore to displace the spoil into the reception shaft. This then has to be manually removed which is time consuming.
Slurry style microtunnelling utilises slurry reticulation to transport spoil removal throughout the installation process. Two lines are fed via a starting shaft along the bore. The pipes are jacked via a hydraulic jacking frame into the hole. Water is forced along the feed pipe to the cutting face where the spoil slurry of rock and mud is forced back along the return pipe. Whilst enjoying a good degree of accuracy, this system requires a structural shaft that needs a massive amount of force to push the pipes. This results in a large, expensive jacking shaft pit that is time consuming to build. The sheer weight and size of the components make them slow to connect and cumbersome to use. If the unit becomes damaged or stuck in the bore, the only method available to retrieve the unit would be to dig down onto the drill head location.
In one form of boring machine shown by US Patent Application No. US2004/0108139 to Davies and corresponding to Australian Patent 2003262292 there is disclosed a micro tunnelling machine having a tunnelling head with a boring bit which is forced in a horizontal direction by an hydraulic thruster. The direction of the head is laser guided. The beam strikes a target in the head and a camera relays an image of the target to an operator located at the tunnel entrance. The operator adjusts the direction by admitting water and draining water from a pair of rams inside the head, which move the boring bit up and down or left and right. A semi automatic version is disclosed in which a microprocessor adjusts the direction until the operator assumes control. In particular the invention is claimed to be a guidance system for the boring head of a micro-tunnelling machine of the type which bores in a selected direction and inclination using laser beam guidance having the endmost part of the drive to the boring bit adjustable in two directions at 90°, wherein, the endmost part of the drive has a target for the laser beam, means to convey an image of the target and the laser strike position thereon to an operator situated remotely from the boring head and input means for the operator to adjust the direction of the endmost part of the drive.
The major approach of the directional control of the disclosed apparatus of US Patent Application No. US2004/0108139 to Davies is to have the drive shaft connected at its end distal to the cutting edge in a manner that allows the drive shaft to move as required and to allow the cutting element to be redirected to correct position as determined by the laser controlled directional system. However this form of apparatus places all the strain on an elongated movable drive shaft retained by cylinders and therefore readily increases the risk of breakage. There is clearly a need to provide an improved system to decrease chance of breakage of the drill head components.
It can be appreciated that present methods of underground tunnelling are cumbersome, inaccurate; and require repeated halting of boring operations due to waste removal and heating effects. Moreover, there is an inherent delay resulting from replacement of parts of conventional boring systems since it usually requires the boring tool to be recovered from the site and returned to the assembly factory. Recovery in itself can be cumbersome and expensive particularly if a new vertical access hole is required to recover the tool. This could damage the road or services under which the bored tunnel is extending. Further present systems are unable to accurately remain on fixed boring direction, which are often needed when a buried obstruction is detected or changing soil conditions are encountered.